A) Field of the Invention
This invention relates to an electrostimulator, and more specifically relates to an electro stimulator which is ideal for assisting or recovering motor functions or strengthening of muscular power of a patient who has had a cerebrovascular accident or the like.
B) Description of the Related Art
There is known “drop-foot” as a sequela of hemiplegia caused by a cerebrovascular accident. Dorsiflexion of an ankle joint is difficult for a patient with drop-foot because of a weakness in the dorsiflexion muscle group and hyperfunction of the plantarflexion muscle group. Therefore, in swing phase of walking, a healthy person can smoothly move a foot forward by dorsiflexion of an ankle joint by contraction of the dorsiflexion muscle group. On the other hand, in case of a patient with drop-foot, a toe touches the ground because of no dorsiflexion of an ankle joint. By this “shuffling gait”, walking becomes hard for a patient with drop-foot.
FIG. 8 is a circuit diagram showing a structure of an electrostimulator according to a first prior art. This structure is disclosed by Japanese Laid-Open Patent No. 2002-331007 (Patent Document 1), and before that, the inventor of the present invention published the first prior art in Muraoka, Y., et. al., EMG-controlled hand opening system for hemiplegia. Proc. 6th Vienna International Workshop on Functional Electrostimulation Basics Technology Application: pp. 255-258, 1998 (Non-Patent Document 1).
Electrodes E11 and E12 are disposed on a muscle belly of muscle from which a muscle activity is picked-up, and an electrode E3 is disposed on an arbitrary position. Even in case of a patient, a faint voluntary myoelectric signal (or electromyography (EMG) signal) is output when the patient tries to contract the muscle, and so the electrodes E11 and E12 are used for detecting the microscopic voluntary myoelectric signal.
With the electrode E3 as a ground electrode, the voluntary myoelectric signal of the target muscle detected by the electrodes E11 and E12 is input to an instrumentation amplifier 11 via protective resistors R11 and R12.
The input of the instrumentation amplifier 11 is limited to about ±0.5V by the diodes D11 and D12 in order not to saturate the instrumentation amplifier 11. Thereafter the output of the instrumentation amplifier 11 is amplified by a multistage amplifier 12 of several stages with a bandwidth of 300-450 Hz to a level which be recognized by a controller 73, and then the amplified signal is taken at a sampling frequency of 1 kHz from an A/D conversion input PIN of the controller 73.
The controller 73 controls a stimulator (stimulus outputting transformer) 74 by outputting a pulse with a width in proportion to amplitude of the voluntary myoelectric signal. A stimulus pulse is a bipolar pulse, and amplitudes of a positive pulse and a negative pulse are the same. The amplitude of the stimulus pulse is about 100V, and the width of the pulse is adjusted to 50 μs-1 ms. The stimulus becomes stronger as the width becomes wider. A cycle of the stimulus pulse is 50 ms, and photoMOS relays SW11 and SW12 are turned on at the timing when the stimulus pulse is applied and conduct the stimulus pulse to the electrodes E11 and E12. When the stimulus pulse is not applied, the photoMOS relays SW11 and SW12 are turned off in order to prevent mixing of noise from the stimulator (stimulus outputting transformer) 74 and simultaneously prevent a short-circuit between the electrodes E11 and E12 via the transformer.
Moreover, the controller 73 controls the timings of turning on and off of the photoMOS relays SW11 and SW12. This prior art uses a transformer as the stimulator (stimulus outputting transformer) 74 which realizes the amplitude of 100V and isolation simultaneously.
FIG. 9 is a circuit diagram showing a structure of an electrostimulator according to a second prior art. This structure is also disclosed by the inventor of the present invention in the Patent Document 2 (Japanese Laid-Open Patent No. 2003-310768). It is different form the first prior art in that electrodes E14 and E15 are commonly connected to one terminal of a stimulator (stimulus outputting transformer) 74 via diode AC switches (diac) D13 and D14 instead of using the photoMOS relays SW11 and SW12, and another terminal of the stimulator (stimulus outputting transformer) 74 is connected to ground, that is, an electrode E16.
By that, the stimulation is performed by two channels of the electrodes E14 and E15 and the electrode E16, and the measurement of the myoelectric signal is performed by one channel of the electrodes E14 and E15. This prior art also uses a transformer as the stimulator (stimulus outputting transformer) 74.
FIG. 10 is a circuit diagram showing a structure of an electrostimulator according to a third prior art. This structure is also disclosed by the inventor of the present invention in the Patent Document 3 (Japanese Laid-Open Patent No. 2004-255104). It is different form the first and the second prior arts in that a boost circuit without a transformer and an H-bridge circuit are used instead of using the stimulator (stimulus outputting transformer) 74 and the photoMOS relays SW11 and SW12 according to the first prior art for down-sizing and weight reduction, and a capacitor C and photoMOS relays SW5 and SW6 are added to share a power source between the boost circuit and the myoelectric signal measurement system.
The third prior art is operated as follows. The capacitor C is charged when no stimulation is given by turning on the photoMOS relays SW5 and SW6. When the stimulation is given, first the photoMOS relays SW5 and SW6 are turned off not to apply the stimulus to the electrode E3, and the electrode E1 which is negative polarity is excited by flowing electricity from the electrode E2 to the electrode E1 by turning on the photoMOS relays SW1 and SW4 while turning off the photoMOS relays SW2 and SW3. Next, the photoMOS relays SW1 and SW4 are turned off and the electrode E2 which is negative polarity is excited by flowing electricity from the electrode E1 to the electrode E2 by turning on the photoMOS relays SW2 and SW3.
FIG. 11 is a circuit diagram showing a structure of an electrostimulator according to a first comparative example. It is different from the third prior art in that the electrodes E1 and E2 just function as recording electrodes, and electrodes E4 and E5 are added as stimulating electrodes. The stimulus is applied to the electrode E3 unless the photoMOS relays SW5 and SW6 are turned off while the stimulus is given also in this structure, and so the above-described technique effectively works.
FIG. 12 is a waveform diagram for explaining a signal process of a controller for detecting a voluntary myoelectric signal while removing stimulus artifact and evoked myoelectric signals. The bottom line represents the stimulus pulse signal consisting of stimulus pulse waveforms impressed every 60 ms. One unit of stimulus pulse signal consists of two stimulus pulse waveforms which are the same stimulus pulse waveforms. Therefore, the stimulus waveforms are renewed every 120 ms.
At an input of the controller 13, a voluntary myoelectric signal is input to the A/D conversion input PIN, sampled at a sampling cycle of 1 ms and converted to a digital signal. The waveform, as shown in the second line from the bottom, is a signal formed by convolving the myoelectric signal including an M-wave with the stimulus signal and the artifact.
The stimulus signal is input at the beginning of the 60 ms cycle. The amplitude of the signal is extremely large comparing to the voluntary myoelectric signal, and the stimulus signal is not necessary for the following signal processes; therefore, the amplitude of the signal is limited not to exceed a predetermined level by two diacs.
Two stimulus waveforms at 60 ms cycle are the same so that the corresponding two artifacts and the M-waves are also the same. Therefore, only the voluntary myoelectric signal can be extracted by cancelling the artifacts and the M-waves out. The signals are not stable from the beginning of the 60 ms cycle for a while (approximately 20 ms); therefore, it becomes possible to extract the component of the voluntary myoelectric signal stably by taking the difference near the end of the cycle (the difference is taken in 15 ms before the end in the below-described example but may be taken in a longer period).